Interpretive Summary: Weed species in agricultural fields result in major economic losses to farmers in the United States. Plant pathogenic bacteria that produce phytotoxins are being considered as biological control agents for weeds due to environmental problems associated with existing chemical controls. However, performance of these plant pathogenic bacteria needs to be improved before they can become commercially viable biological control agents of weeds. One performance issue with these bacteria is the inconsistent production of virulence factors that ultimately kill weed plants. This study was undertaken to determine the regulatory mechanisms phytotoxin-producing, plant pathogenic Pseudomonas syringae bacteria use to respond to environmental conditions in agricultural fields and cause disease. Little is known regarding the impact of various environmental conditions found in agricultural fields on the production of these virulence factors. In this study we determined the role of the posttranscriptional regulatory protein RsmA in disease and in production of the underpinning virulence factors that lead to disease by these phytotoxin-producing P. syringae bacteria. Once regulatory mechanisms that are intimately involved in disease production are understood strategies can be developed that ensure consistent production of virulence factors and disease. This information will be useful to scientists devising strategies to improve biological control of weeds through consistent production of virulence factors by plant pathogenic, phytotoxin-producing bacteria.

Technical Abstract:
The GacS/GacA two-component system functions mechanistically in conjunction with the global post-transcriptional regulator RsmA to allow pseudomonads and other bacteria to adapt to changing environmental stimuli. Analysis of this Gac/Rsm signal transduction pathway in phytotoxin-producing pathovars of Pseudmonas syringae is incomplete, particularly with regard to rsmA. Our approach in studying rsmA was to overexpress rsmA in P. syringae strains through introduction of pSK61, a stably maintained plasmid constitutively expressing this gene. Disease and colonization of plant leaf tissue were consistently diminished in all P. syringae strains tested (pv. phaseolicola NPS3121, pv. syringae B728a, and BR2R) when containing pSK61 relative to these isolates containing the empty expression vector pME6031. Phaseolotoxin, syringomycin, and tabtoxin were also not produced in these strains, respectively, when containing pSK61. Protease and pyoverdin production and swarming were also diminished in all of these strains when containing pSK61. In contrast, alginate production, biofilm formation, and the hypersensitive response were diminished in some, but not all, of these isolates under the same conditions. These results indicate that rsmA is consistently important in the overarching phenotypes disease and epiphtyic colonization, but that its role varies with pathovar in certain underpinning phenotypes in the phytotoxin-producing strains of P. syringae.